I am not familiar with clustering....
Would you run a mod_perl based web site on a cluster? Isn't the point of a cluster to make a group of machines appear to be one? If so, how is that beneficial for web services?
All large web sites run on multiple servers, for redundancy and performance reasons. You put a load-balancer of some kind in front of them so that the group of machines has a single URL from the outside world. This is called clustering.
I believe you'll be just fine with either Sun or PC hardware. Large sites -- including huge ones like Yahoo, Amazon, and AOL -- usually prefer PC hardware these days because of the value they get for their money.
- Perrin
